1. Field of the Invention
The present invention relates to electronically actuated luminescent displays in which the images of alphabetic characters are produced in response to electronic codes.
2. Description of the Prior Art
In the past, various types of luminescent displays for imaging alphabetic characters have been commercially available. In many commercial devices, such as electronic calculators and the like, the output displays are formed by multiple character, plural segment arrays of light emitting diodes. In a simple character display, seven of such diodes are arranged generally in the form of the numeral "8". Selected combinations of these light emitting diodes can be actuated at each character position to provide a visual output of a sequence of characters. Such a seven segment display is quite adequate for imaging arabic numeral characters, but is incapable of providing images of alphabetic characters due to an inability to provide a recognizable pattern of illumination associated with each character.
Because of the relative complexity in the display of alphabet characters, as opposed to arabic numerals, more complex display arrangements have been necessary for this purpose. For example, in a conventional dot matrix display a plurality of contiguous electrically energizable light emitting elements are provided in closely packed ordered arrangement, usually a rectilinear matrix. These elements may be light emitting diodes, vacuum tube electrodes, and other types of luminescent devices. When energized in appropriate patterns, these light emitting elements can be used to form images of both upper and lower case alphabet characters.
One commercially available dot matrix display device is marketed by ISE Electronics Corp. as the Noritake "itron" type DC 165A2 Display. This display is a dot matrix display 16 characters in length. The character positions are sequentially addressed in time multiplexed arrangement. This particular display, like many commercial devices, has connections for a segment driver system responsive to a standard ASCII code for upper and lower case characters and symbols. Each character is comprised of a rectangular matrix seven dots in height and five dots in width, for a total of 35 dots. The overall height of the seven dots is seven millimeters from bottom to top. This particular device employs gas fluorescent luminescent elements. Like many commercially available dot matrix displays, this device is capable of imaging only numbers and upper case English language alphabet characters without producing significant distortions. When lower case characters are imaged there are severe distortions, both in the image of the lower case characters themselves, and also in the relative vertical alignment of lower case characters with respect to other characters in the display. This may be attributed to the loss of resolution that accompanies the display of lower case characters because of the horizontal rows of dots that are unavailable to display the main bodies of letters, but which must instead be used to image ascenders and descenders.
The problem that arises with this and other conventional displays in imaging lower case characters stems principally from the fact that unlike upper case English language alphabet characters, lower case letters are not correctly depicted as being of uniform height. That is, some lower case letters such as the letters "g", "p" and "q" include descending portions. These letters, when correctly depicted, include portions which extend below a base line above which the main bodies of all alphabet characters, both upper and lower case, are positioned. Similarly, other lower case alphabet characters, such as the letters "f", "l" and "t", and all upper case letters, include ascending portions which rise vertically above an area encompassing the main body of all lower case English language alphabet letters. Still other lower case letters, such as "a", "c" and "x" lie entirely within the vertical confines of an area having its lower limit at the imaging base line and extending upward only a portion of the overall height of capital letters and lower case letters having ascending portions.
Ideally, all lower case English language alphabet characters should be positioned and aligned directly atop a straight, horizontal base line. When these lower case letters appear side by side, as occurs in displaying images of words, the descending portions of letters having descenders should extend below the base line. Ascending portions of letters having ascenders should extend above the main body area common to all lower case letters. This main body area begins at the baseline and extends upward between about 50% and about 60% of the maximum letter height. In practice, however, conventional displays have been unable to correctly produce images of lower case letters in proper horizontal alignment without an unduly large and complex display. No 5 by 7 dot matrix display has heretofore existed which will acceptably display lower case letters.
In the Motorola Memory Data Book, published by Motorola Corporation, 1977, pages 3-27 through 3-40, a rather complex display format is described. The characters are actually displayed within a 7 by 9 rectilinear dot matrix framework. However, a character display 16 bits in height is required to properly position lower case letters. With this 7 by 16 dot display, a seven by 9 dot rectilinear portion is actuated in response to a standard ASCII code, and is shifted a specified number of rows within the seven by sixteen dot matrix. This achieves proper positioning of the lower case characters, but only with additional ROM capacity to perform the necessary shifting and then only with the oversize display matrix. Furthermore, the inordinately great height of the dot matrix required (7 by 16) prohibits the use of such a display within many compact electronic instrumentation packages.
A very similar type of display system is described in connection with the National Semiconductor DM8678 bipolar character generator, appearing on pages 9-22 to 9-31 of the Digital Data Handbook, published by National Semiconductor Corporation, 1977. This arrangement contemplates a visual CRT display in either a 7 by 9 or 5 by 7 character font matrix. As with the Motorola system, the 5 by 7 or 7 by 9 font is shifted several rows for lower case characters having descenders. Also, in the 5 by 7 matrix the characters produced are distorted because the ascenders and descenders have been made too short in order to maximize the detail of resolution in the body portion of the character. For example, the lower case letters "g" and "y" are quite distorted.
All of the conventional character displays heretofore available have severe limitations in providing images of lower case English language letter characters. The character displays of conventional five by seven and seven by nine dot matrix displays have poor resolution. This defficiency in resolution arises because a dot density which is sufficient to image an upper case character that extends the entire height of the display format is insufficient to provide the greater detail required in lower case characters in the area immediately above the base line. The alternative systems which provide a larger matrix have severe cost penalties both in the intricacy of the display construction, and in the display driving address system. In contrast to 35 pinouts, which are all that are necessary to provide a five by seven dot matrix display, an increased number of pinouts are required to accomodate the additional rows of dots through which lower case characters with descenders are shifted.
As an alternative to dot matrix displays, configured types of displays can also be employed to provide images of letters. The electrodes of gas discharge displays, for example, can be configured to improve resolution in the display of lower case characters. That is, electrodes immediately above the character base line are provided in a greater density and with more precise geometry than can be achieved with conventional dot matrix display systems. However, gas discharge displays which have heretofore been constructed in this manner are highly specialized displays and cost considerably more than dot matrix displays.